In the forthcoming year we shall be continuing our experiments on the role of urea in the rabbit renal concentrating process: 1) effects on renal concentrating ability of acute urea administration in rabbits fed low and high protein diets; 2) effects of dietary protein intake on concentrating ability; 3) intra renal distribution of urea, sodium and potassium in these rabbits. We shall also study the role of brief periods of starvation on the renal handling of urea in sheep.